1. Field of Invention
The present invention relates to a method of ultrasonic welding with which a low strength conductive member, such as a flat conductor, can be reliably welded to another bonding member without creating a welding scar inflicted by the horn tip or the anvil.
2. Brief Description of the Prior Art
In an ultrasonic welding machine such as shown in FIG. 5, at the front end of the horn of a ultrasonic welding machine 25, a horn tip 26 is secured to function as one portion of a welding jig and an anvil 28 is secured on a base table 27 facing opposite the horn tip 26, to function as another portion of the welding jig. A bonding member is set on the anvil 28 and when the horn 29 is lowered to come in contact with the bonding member, the horn tip 26 ultrasonically vibrates in the lengthwise direction so that the individual bonding members can be connected to each other by welding.
As the bonding members, a copper plate member 30, a flat conductor of a flat wire 31 or copper foil may be used, for instance. As shown in FIG. 6, at another end of the copper plate member (joint member) 30, a stranded conductor 34 of a electrical wire 33 is welded in a similar manner and, with this, the flat wire 31 and the electrical wire 33 are electrically connected.
At the lower end surface (working surface) of the horn tip 26, indented portions 35 and projected portions 36 are formed to form cross shapes, as shown in FIGS. 7 and 8, for instance, and it is possible to communicate ultrasonic waves to the bonding members efficiently (Japanese Utility Model Laid Open No. 1989-139986).
However, when welding a low strength conductive member such as the flat conductor 32 or a copper foil using the horn tip 26 provided with the indented portions 35 and projected portions 36 forming cross shapes described above, since the projected portions 36 come in contact with the low strength conductive member 32 diagonally relative to the lengthwise direction of the low strength conductive member 32 and the projected portions run at right angles to each other, the flat conductor 32 tends to develop an indented welding scar 37 relative to the copper plate member 30 such as a bus bar, as shown in FIG. 9. It has been confirmed, based upon measurements of the peeling strength made by pulling the bond apart vertically relative to the bonding surface 38, that in such a case, peeling does not occur starting from the bonding surface 38 but rather a rupture tends to occur starting at the root 39 of the bonding portion, due to the welding scar 37, reducing the strength of the low strength conductive member such as the flat conductor 32.